An experimental investigation of radon diffusion in an anhydrous andesitic melt at atmospheric pressure: Implications for radon degassing from erupting magmas

摘要

We have measured the diffusivity of radon in a dry natural andesitic melt at atmospheric pressure and at temperatures ranging between 1350 deg C and 1500 deg C. The procedure is based on the determination by gamma-ray spectrometry of radon losses undergone by a disk-shaped melted sample during heating. A theoretical treatment of radon diffusion allowed us to relate these losses to diffusivities and values of D_(Rn) can be described by the following Arrhenius equation: D_(Rn)=D_0 centre dot exp(-E_a/RT), with D_0 = 1.5 10~4 m~2 centre dot s~(-1) and E_a = 466 kJ centre dot mol~(-1). Our results confirm that the larger the ionic radius of the noble gas, the higher the frequency factor D_0 and the activation energy E_a. At a temperature of 1225 +- 25 deg C the diffusivity of radon approaches that of other heavy noble gases (log D_(Rn) = -12.0 +- 0.1 where D_(Rn) is in m~2 centre dot s~(-1)). From the present study it is demonstrated that neither diffusion processes, which are too slow and balanced by radioactive ingrowth of radon from its parent, nor exsolution of pure radon bubbles, because of its exceedingly low abundance in the magma, can be responsible for the complete radon depletion observed in erupted lavas. The exsolution of water and other major gas phases acting as carriers of trace gaseous species appears to be the main cause for radon as well as other noble gases degassing.